POLYMORPHISM

POLYMORPHISM

A polymorphism is a genetic variant that appears in at least 1% of a population. Examples: the human ABO blood groups, the human Rh factor the human major histocompatibility complex (MHC). By setting the cutoff at 1%, it excludes spontaneous mutations that may have occurred in — and spread through the descendants of — a single family.

Protein Polymorphisms: 

All the examples above are of the protein products of alleles. These can be identified by: serology; that is, using antibodies to detect the different versions of the protein and electrophoresis; if amino acid changes in the protein alter its net electrical

charge, it will migrate more or less rapidly in an electrical field.

Enzymes are frequently polymorphic:

A population may contain two or more variants of an enzyme encoded by a single locus. The variants differ slightly in their amino acid sequence and often this causes them to migrate differently under electrophoresis. By treating the gel with the substrate for the enzyme, its presence can be visualized. Electrophoretic variants of an enzyme occurring in a population are called allozymes.

Restriction Fragment Length Polymorphisms (RFLPs): 

Proteins are gene products and so polymorphic versions are simply reflections of allelic differences in the gene; that is, allelic differences in DNA. Often these changes create new — or abolish old — sites for restriction enzymes to cut the DNA. Digestion with the enzyme then produces DNA fragments of a different length. These can be detected by electrophoresis.

Single Nucleotide Polymorphisms (SNPs): 

Developments in DNA sequencing now make it easy to look for allelic versions of a gene by sequencing samples of the gene taken from different members of a population (or from a heterozygous individual). Alleles whose sequence reveals only a single changed nucleotide are called single nucleotide polymorphisms or SNPs.

SNPs can occur in noncoding parts of the gene so they would not be seen in the protein product. might not alter the cutting site for any known restriction enzymes so they would not be seen by RFLP analysis.

Copy Number Polymorphisms (CNPs): 

Genetic analysis (using DNA chips and FISH) has revealed another class of human polymorphisms. These copy number polymorphisms are large (thousands of base pairs) duplications or deletions that are found in some people but not in others. On average, one person differs from another by 11 of these. One or more have been found on most chromosomes, and the list is probably incomplete

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While most of this DNA is non-coding, functional genes are embedded in some of it. How, or if, the person adapts to the resulting change in gene number is unknown

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